you write
I use two sealed lead acid agm batteries rated at 5.1 ah each, configured in series, to make 10.2 ah. This battery with my alternator will carry my system successfully through the full range of RPM's.
As I understand you, you connected two 6 Volt in series, doubling the system voltage (12V). But this configuration imho would not increase the Ah value, but the energy capacity (Watt-hours).
You would double the Ah capacity by connecting two 12 Volt batteries in parallel.
Hans